1. Field of the Invention
The present invention relates to an apparatus and method for detecting the friction characteristics, or more in particular to a friction characteristics detecting apparatus for controlling the wheel regulating force to a value in the vicinity of the peak friction force on the basis of the resonant characteristics of a wheel resonant system including the road surface and at least a wheel and detecting the friction characteristics on the basis of the regulating force thus controlled.
2. Description of the Related Art
Some techniques for measuring the friction force or the friction coefficient and techniques for controlling the friction force by detecting the friction characteristics are described below.
In a model shown in FIG. 7, the maximum friction force F.sub.max generated between an object 1 of mass M and a surface to be measured 2 is given as EQU F.sub.max =.mu..sub.stat .multidot.W
where W is a load (=Mg, where g is the acceleration of gravity) and .mu..sub.stat is the static friction coefficient. In the case where a force F.sub.ext applied from an external source is equal to or below the maximum friction force F.sub.max, the static state is retained. Once the external force F.sub.ext exceeds the maximum friction force F.sub.max, however, the object abruptly begins to slip. The friction force F.sub.trans involved is expressed as EQU F.sub.trans =.mu..sub.trans .multidot.W
where .mu..sub.trans is the dynamic friction coefficient.
In this way, the object remains totally unchanged until the external force F.sub.ext exceeds the maximum friction force F.sub.max, and the characteristics undergo a great change immediately upon exceeding the maximum friction force. It is therefore very difficult to determine whether the object is just about to slip or not.
In conventional systems for measuring the friction force, an object is caused to slip by applying an external force thereto in advance and the maximum friction force F.sub.max involved is measured. Further, the maximum friction force F.sub.max is divided by the weight of the apparatus thereby to measure the static friction coefficient .mu..sub.stat.
Also, in the field of casting industry, there is a technique for measuring the friction force, wherein a die for continuous casting is fixedly supported on a vibrating table and coupled to a vibration source through a vibrating beam. The vibrating beam is oscillated about a supporting point by the operation of the vibration source, and the friction force exerted between the die and the cast piece is measured when drawing the cast piece while vibrating the die. These techniques take advantage of the fact that the friction force between the die and the cast piece affects the load of the vibration source, and the characteristics of the die vibrating system are expressed by a transfer function, so that the friction force between the die and the cast piece is measured on the basis of the transfer function. These techniques for measuring the friction force in the field of casting engineering include the one disclosed in Japanese Patent Application Laid-Open (JP-A) No.4-84652 as a technique securing an improved calculation speed and accuracy of calculation.
In the technique disclosed in Japanese Patent Application Laid-Open (JP-A) No.4-84652, the friction force exerted between a die and a cast piece when drawing a cast piece from the die under vibration is calculated on the basis of the oscillation torque exerted on the die under vibration and the displacement generated in the die by the vibration according to a state space model obtained by formulating the portion nearer to the die than the supporting point of the vibrating beam by the mode separation method. In order to accurately determine the oscillation torque acting on the die, the exciting force generated by a vibration source such as a vibrating cylinder and the stress generated in the vibrating beam at a portion nearer to the vibration source than the supporting point of the vibrating beam are detected, and the exciting force thus detected is corrected by the stress generated in the vibrating beam, thereby calculating the oscillation torque. In this way, according to this technique, a high-speed calculation is accomplished by modeling the portion between the supporting point of the vibrating beam and the die to be vibrated in a simplified way as a flexible beam for vibrating a concentrated mass including a vibrating table and a die by the oscillation torque exerted on the supporting point. Further, in calculating the oscillation torque, the exciting force is corrected by the stress of the vibrating beam, thereby making it possible to measure the friction force accurately.
In the field of automotive control engineering, a technique for measuring the friction coefficient between the wheels and the road surface and controlling the operation on the basis of the friction coefficient is presented by an electronically-controlled power steering apparatus disclosed in Japanese Patent Application Laid-Open (JP-A) No.4-230472.
The method for measuring the friction coefficient disclosed in Japanese Patent Application Laid-Open (JP-A) No.4-230472 is such that a vibration signal is applied to a solenoid valve from a controller, whereby the rear wheels are manipulated periodically at a steering angle equivalent to +/-1 mm and a frequency of 2 Hz, and the reaction force against the self-aligning torque and the cornering force generated in the rear wheels by this periodic steering operation are detected by a reaction force sensor such as a load cell. The cornering power and the self-aligning power are calculated on the basis of the reaction force thus detected, and the road surface friction coefficient is measured from the calculation result on the basis of the relation between these powers and the road surface friction coefficient.
Also, an anti-lock brake control apparatus is known as a technique for preventing the wheels from locking and slipping even under a suddenly braked condition by estimating the friction characteristics between the road surface and the wheel and controlling the braking force in such a manner that the contact surface reaches the stage just about to slip.
Suppose that the braking force is progressively applied when the vehicle is running at a predetermined velocity. A slip occurs between the wheel and the road surface. The friction coefficient .mu. between the wheel and the road surface, however, is known to change as shown in FIG. 9 with respect to the slip ratio S expressed by equation (1) below. EQU S=(v.sub.v* -v.sub.w)/v.sub.v* ( 1)
where v.sub.v* is the velocity of an actual vehicle body, and v.sub.w is the wheel velocity.
With this .mu.-S characteristic, the friction coefficient .mu. assumes a peak value for a predetermined slip ratio (region A2 in FIG. 9).
In the conventional anti-lock brake control apparatus, therefore, the slip ratio is detected from the vehicle body velocity and the wheel velocity, and the braking force is controlled in such a manner as to secure a slip ratio associated with the peak value of the friction coefficient .mu..
With the above-mentioned conventional friction force detecting apparatus, however, in the event that the slip is not allowed or the friction coefficient undergoes a change, it is very difficult to detect in real time which state the friction characteristics are in at present.
The friction force measuring apparatus disclosed in Japanese Patent Application Laid-Open (JP-A) No.4-84652 assumes a linear model of a die approximated by the concentrated mass under vibration by a oscillation torque and calculates the friction force affecting the die vibration on the basis of only the displacement of the die and the oscillation torque. The problem therefore is that the measurement is liable to be affected by noises, etc. Under the conditions failing to satisfy the foregoing assumption, accurate measurement of the friction force is impossible with this model, and therefore the range of application is very limited. Even if a model meeting the conditions of assumption can be constructed, complicated calculations are often required depending on the model. In such a case, a new problem is posed that the friction force cannot be calculated in real time.
Also, in the method for measuring the coefficient of friction disclosed in Japanese Patent Application Laid-Open (JP-A) No.4-230472, the cornering force and the like are required to be generated by periodically steering the wheels and the reaction of the wheels against the cornering force must be detected. This poses the problem of a complicated measuring system. Further, since the relation between the cornering power or the like and the road surface coefficient of friction is assumed with a predetermined model, the system easily succumbs to noises.
With the conventional anti-lock brake control apparatus, the friction coefficient .mu. between the tire of a running vehicle and the road surface undergoes a constant change and considerable disturbance and noises are generated while the vehicle is running. The slip ratio associated with the maximum friction coefficient .mu., therefore, also undergoes a change, thereby making it very difficult to accomplish a proper brake control.
The object of the present invention, which has been developed in order to obviate the above-mentioned problems of the prior art, is to provide a friction characteristics detecting apparatus having a simple configuration for a wide range of applications, in which the friction characteristics are not detected only on the basis of the displacement response or the vibration characteristics or the like for an assumed model depending to a large measure on the system configuration and conditions, but the braking force applied to the wheels is controlled in such a manner that the friction force assumes a peak value on the basis of the resonant gain quickly reflecting the friction characteristics between the wheel and the road surface, and the friction characteristics are detected based on the particular braking force. Thus it is possible to accurately detect the friction characteristics even in the case where noises are considerable and the fiction characteristics are constantly undergoing a change.